The long term objectives are to understand the physiologic changes that occur during the growth and development of the respiratory system as well as the changes produced by injury to the lungs early in life. In infancy, the two primary insults to the lungs occur from lower respiratory tract infections (LRI) and the medical support required or premature infants which results in bronchopulmonary dysplasia (BDP). There has been increasing concern that these early pulmonary insults may have chronic sequellae and contribute to the development of chronic obstructive airways disease in adults. Pulmonary function testing in infants has been limited by patient cooperation and the lack of easily performed, non-invasive procedures. Non-invasive techniques for obtaining partial expiratory flow-volume curves and total respiratory system compliance have recently been described for use in infants. This investigator proposes to employ these techniques in the following studies: 1) classify BPD in terms of the degree of pulmonary dysfunction. 2) evaluate bronchodilator responsiveness in infants with BPD and with LRI. 3) evaluate prospectively the relationships of LRI's pulmonary function and bronchial reactivity during the first year of life. 4) evaluate the relationship between the progression of pulmonary dysfunction and bronchidilator responsiveness as infants progress from respiratory distress syndrome to BPD.